1. Field of the Invention
The present invention relates to an automatic transmission for a motor vehicle. More particularly, it relates to the control system of an automatic transmission wherein power delivered from an engine is transmitted to a gear shift mechanism through a damper, and the power is transmitted from the gear shift mechanism to driving wheels through a starting clutch.
2. Description of the Prior Art
There has heretofore been known an automatic transmission of so-called "friction start type " which starts a motor vehicle by bringing a starting clutch into engagement.
By way of example, the official gazette of Japanese Patent Application Laid-open No. 165149/1992 discloses an automatic transmission wherein power delivered from an engine is transmitted through a damper to a gear shift mechanism which consists of a stepless transmission device and planetary gears for switching forward/reverse movements, and the power is transmitted from the gear shift mechanism to driving wheels through a starting clutch.
With the prior-art automatic transmission, the starting clutch should preferably be held in complete or full engagement from the low rotational speed of the engine in order to enhance fuel consumption. It is recommendable for such complete engagement to bestow a small spring constant on the damper for absorbing a torque fluctuation, and to shift the resonance point (namely, resonance frequency) of a system including the damper, into the speed zone of the engine lower than the idling rotational speed thereof.
However, when the damper of the small spring constant is employed in this manner, the rotational speed of the engine traverses the resonance point in the engine starting operation of the motor vehicle. This poses the problem that a vibration of large amplitude takes place in the motor vehicle.